Pedantically, I'm not sure if there are any extra physical chips on the Wii U for backwards compatibility, at least for the major bits. On the CPU side, the Wii U uses a weird triple core PowerPC 750 (aka G3) setup, even though the 750 was a) ancient and b) not designed for SMP, in order that it could be used for backwards compatibility with the previous 750 powered units. On the GPU side it does still have the old GC / Wii "GX" GPU as a separate unit, but it's just on the same chip as the modern GPU.